Long Answer Questions

With respect to the Hall Heroult process related to the extraction of aluminium, justify the following:

(a) Powdered Coke is sprinkled over the electrolytic mixture undergoing electrolytic reduction.

(b) Graphite anodes are continuously replaced during the electrolysis.

(c) Cryolite and fluorspar must be added to the electrolytic mixture.

Answer

(a) The layer of powdered coke is sprinkled over the electrolytic mixture as:

1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
2. It minimizes or prevents heat loss by radiation.

(b) The graphite anodes are continuously replaced during the electrolysis because:

1. The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode.
2. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet.
   2C + O₂ ⟶ 2CO
   2CO + O₂ ⟶ 2CO₂

(c) Cryolite and fluorspar are added to the electrolytic mixture because their addition:

1. lowers the fusion point of the mixture i.e., mixture fuses around 950°C instead of 2050°C.
2. enhances the mobility of the fused mixture by acting as a solvent for the electrolytic mixture.
3. enhances the conductivity of the mixture since, pure alumina is almost a non-conductor of electricity.

Write complete and balanced equations for the reactions occurring in the following cases:

(a) Passing dry ammonia gas over heated lead oxide placed in a combustion tube to produce a silvery grey metal.

(b) When concentrated nitric acid is reacted with zinc to produce a reddish-brown gas.

(c) When concentrated sulphuric acid oxidises sulphur to produce a gas which turns acidified potassium dichromate paper green.

Answer

(a) NH₃ + 3PbO ⟶ 3Pb + 2H₂O + 2NO₂

(b) Zn + 4HNO₃ ⟶ Zn(NO₃)₂ + 2H₂O + 2NO₂

(c) 2H₂SO₄ + S ⟶ 3SO₂ + 2H₂O

Give balanced equations for the conversions A, B and C.

$\text{Zn} \longrightarrow {\overset{\text{A}}{\text{ZnCl}_2}} \longrightarrow {\overset{\text{B}}{\text{Zn(OH)}_2}} \longrightarrow {\overset{\text{C}}{\text{ZnSO}_4}}$

Answer

(A) Zn + 2HCl ⟶ ZnCl₂ + 2HCl

(B) ZnCl₂ + 2NaOH ⟶ Zn(OH)₂ + 2NaOH

(C) Zn(OH)₂ + H₂SO₄ ⟶ ZnSO₄ + 2H₂O

Rohan wants to electroplate a spoon with nickel.

(a) To which electrode should he connect the article to be electroplated?

(b) Write the equation for the reaction that will occur at the cathode.

(c) What should the anode be made up of?

Answer

(a) The article to be electroplated is placed is made as cathode. This is because metal cations from the electrolyte will move toward the cathode and get reduced and deposited on the spoon.

(b) Ni²⁺ + 2e^- ⟶ Ni

(c) The anode should be made of pure nickel metal. During electroplating, nickel atoms from the anode dissolve into the solution as Ni²⁺ ions, maintaining a steady concentration of nickel ions in the electrolyte.

Amit found that 30g of a gas occupied 1000 c.c at STP.

(a) What will the gram molecular weight and the vapour density of this gas be?

(b) How many molecules of this gas will be present in 44.8 l of it?

Answer

Given,
Mass of gas = 30g
Volume = 1000 c.c = 1dm³ = 1 liter

We know one mole of gas at STP occupies 22.4 l

1 litres of gas at S.T.P. has mass = 30 g

∴ 22.4 litre (molar volume) has mass

= $\dfrac{30}{1}$ x 22.4

= 672 g/mol

Vapour density = $\dfrac{\text{Molecular mass}}{2}$

= $\dfrac{672}{2}$

= 336

(b) 22.4 litres of the gas contains 6.023 x 10²³ molecules

So, 44.8 litres of the gas contain 2 x 6.023 x 10²³

= 12.046 x 10 ²³ molecules

An element X combines with oxygen to form an oxide X₂O₃. This oxide is a good conductor of electricity and can be reduced to its metal only by electrolysis.

(a) Write the equation for the reaction formed when the oxide (X₂O₃) combines with hydrochloric acid.

(b) How many valence electrons are present in the outermost shell of X?

(c) Will element X undergo oxidation or reduction?

Answer

Element X is a metal, and its oxide is X₂O₃ so X must have a valency of 3 (X³⁺). The oxide is a good conductor of electricity. Since it requires electrolysis to extract X, it must be a very reactive metal.

(a) The oxide (X₂O₃) is reacting with hydrochloric acid, so we can say the oxide is ionic and basic in nature. The reaction will give salt and water.

X₂O₃ + 6HCl ⟶ 2XCl₃ + 3H₂O

(b) Since X forms X³⁺, it must lose 3 electrons, meaning it has 3 valence electrons.

(c) To form X³⁺, element X must lose 3 electrons and loss of electron is oxidation.

Observe the reactions given below and answer the following questions:

(a) Identify compound P.

(b) Give the chemical formula of Z.

(c) Write the reaction taking place between the identified compound P and sulphuric acid.

(d) Name compound Y.

Answer

(a) NaOH

(b) C₂H₅OH

(c) NaOH + H₂SO₄ ⟶ Na₂SO₄ + H₂O

(d) Sodium plumbite

Study the information given in the table below and answer the questions that follow. (Note- the letters do not represent the actual symbols of the elements)

(a) Explain why element X has highest ionisation energy.

(b) To which period does Z belong?

(c) Draw the electron dot structure of the compound formed between Z and oxygen.

Answer

(a) Element X has the highest ionisation energy because it has the smallest atomic size, fewer electron shells, and its valence electrons experience a stronger attraction to the nucleus.

(b) Z has the configuration of 2,8,8,2, electrons occupying K,L,M and N shells. Hence, Z belongs to Period 4.

(c) The electron dot structure of the compound formed between Z and oxygen is shown below:

A student prepared a Potassium sulphite solution in the lab and added few drops of barium nitrate solution to it. He observed a white precipitate being formed in the test tube. On addition of dilute hydrochloric acid to the white precipitate and mixing it, he observed that the precipitate disappeared.

(a) Name the white precipitate.

(b) Write a balanced chemical equation for the reaction between dilute hydrochloric acid and the white precipitate.

(c) Name the gas evolved in the above reaction.

Answer

(a) When the student mixed potassium sulphite solution (K₂SO₃) with barium nitrate (Ba(NO₃)₂), the white precipitate of Barium sulphite (BaSO₃) is formed.

K₂SO₃ + Ba(NO₃)₂ ⟶ BaSO₃ + 2KNO₃

(b) When white precipitate of BaSO₃ reacts with dilute HCl, it dissolves, forming barium chloride (BaCl₂), water, and sulphur dioxide (SO₂) gas.

BaSO₃ + 2HCl ⟶ BaCl₂ + H₂O + SO₂ ↑

(c) From the above reaction, the gas produced is sulphur dioxide (SO₂).

The empirical formula of a hydrocarbon is C₂H₃. The hydrocarbon has a relative molecular mass of 54. (At wt: H = 1, C = 12)

(a) What is the molecular formula of the hydrocarbon?

(b) Draw the structural formula of the hydrocarbon.

(c) Give the general formula of the hydrocarbon.

Answer

(a) Empirical formula = C₂H₃

Relative molecular mass = 54

$\text{n} = \dfrac{\text{Molecular weight}}{\text{Empirical formula weight}} \\[0.5em] = \dfrac{54}{27} = 2$

∴ Molecular formula = n[E.F.] = 2[C₂H₃] = C₄H₆

(b) The structural formula of butyne is shown below:

(c) The hydrocarbon C₄H₆ is an alkyne, and the general formula for alkyne is C_nH_2n-2.

Study the given figure and answer the given questions:

(a) Identify the application of electrolysis demonstrated above.

(b) Which metal is protected in the above process?

(c) Why should the metal be protected?

Answer

(a) Electroplating

(b) Iron

(c) Iron tools and instruments are often electroplated with zinc to protect against rusting.

Nita's father bought a basket of ripe mangoes. While opening it she found a small sachet containing a white crystalline powder along with the mangoes. She was told that it is a chemical that releases a gas when it comes in contact with moisture, that induces ripening of fruits.

(a) Name the chemical powder in the sachet.

(b) Name the gas.

(c) Give a balanced chemical equation for the reaction that results in the evolution of this gas.

Answer

(a) Calcium carbide (CaC₂)

(b) Acetylene/Ethyne.

(c)

$\underset{\text{calcium carbide}}{\text{CaC}_2} + \underset{\text{water}}{2\text{H}_2\text{O}} \longrightarrow \underset{\text{ethyne or acetylene}}{\text{C}_2\text{H}_2} + \underset{\text{calcium hydroxide}}{\text{Ca(OH)}_2}$

Atomic number of element M is 12 and it forms an ionic compound with element L.

(a) Which of the following atomic numbers will match L?
i. 14
ii. 10
iii. 8

(b) What is the name given to the members of the group to which element M belongs?

(c) Draw the electron dot structure of the compound formed between M and L.

Answer

Atomic number of element M = 12

Electronic configuration of element M = 2, 8, 2

∴ Element M is a metal with valency 2, it will form M²⁺ ion

(a)

The correct match for L will be (iii), because M can form an ionic compound with L (M²⁺ + L^2- = ML)

(b) As element M contains 2 valenced electrons, it belongs to Group 2 of the Periodic Table and group 2 elements are called Alkaline Earth Metal.

(c) The electron dot structure of the compound formed between M and L is shown below:

When two dry gases, oxygen and X, are passed over heated platinum, reddish-brown fumes are seen in the receiving flask, as shown in the figure.

(a) Name the gas X.

(b) Give equation(s) for the reaction(s) that resulted in the formation of brown fumes.

Answer

(a) The gas X is Ammonia (NH₃).

(b) 4NH₃ + 5O₂ $\xrightarrow[800 \degree \text{C}]{\text{Pt.}}$ 4NO↑ + 6H₂O + Δ
     2NO + O₂ ⟶ 2NO₂ [brown gas]